1. Field of the Invention
The present invention relates to a zoom lens, and particularly to a zoom lens for projection applicable to a projection-type display apparatus.
Further, the present invention relates to a projection-type display apparatus including such a zoom lens for projection.
2. Description of the Related Art
Recently, projection-type display apparatuses, such as a transmission-type or reflection-type liquid crystal display apparatus and a DMD display apparatus, using light valves became widely used. Projection lenses used in such kinds of projection-type display apparatuses generally need to have long back focus. Further, an entrance pupil of such a projection lens viewed from a reduction side (light valve side) needs to be located sufficiently far. In other words, the projection lens needs to have telecentricity on the reduction side. Further, the projection lens needs to have a variable magnification function (zoom function), considering the portability of the apparatus and the adaptability of the apparatus to setting conditions. Further, the projection lens needs to have a wide angle of view to satisfy a requirement of projection onto a large screen from a short distance.
When the angle of view of the projection lens is tried to be widened, a problem of an increase in the screen-side lens diameter and a problem of a fluctuation of curvature of field caused by focusing when a projection distance is changed (from close-up to infinity) arise.
Conventionally, zoom lenses disclosed in Japanese Unexamined Patent Publication No. 2008-257005 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2011-013657 (Patent Document 2) are well known, as zoom lenses for projection that can cope with the aforementioned problems. In the zoom lens disclosed in Patent Document 1, a first lens group, which is located on the most magnification side, is divided into two sub-lens-groups. A fluctuation of curvature of field caused by focusing is suppressed by changing a distance between the two sub-lens-groups during focusing. Meanwhile, in the zoom lens disclosed in Patent Document 2, a fluctuation of curvature of field caused by focusing is suppressed by moving the second most-magnification-side lens group. Further, the zoom lens disclosed in Patent Document 2 is configured in such a manner that four lens groups are moved during magnification change.